The present invention generally relates to metal treatments for adhesive-bonded metal substrates and more specifically to silane triol capped epoxy-amine adhesion promoters for adhesive-bonded metal substrates.
Fatigue of aircraft structure is one of the problems limiting the useful life of aircraft, including military aircraft. Adhesively bonded repairs and/or reinforcements are key approaches for maintaining aging aircraft, especially as a remedy for fatigue cracks. Adhesive bonding is also important for maintaining other Department of Defense weapons systems. Bonded repairs provide substantial cost savings and reduced aircraft downtime when compared to component replacement. Bonded repairs also have several advantages over traditional repair approaches using mechanical, including improved structural efficiency, improved fatigue life due to the elimination of fastener holes, and weight savings.
Currently available paste adhesives often do not provide satisfactory bonding to metal surfaces in the absence of metal treatment. Metal treatment prior to bonding is a key factor for both the initial adhesion of a paste adhesive and for long-term durability. Current metal prebond surface preparations, however, especially for on-aircraft repair, are inconvenient to use and/or often do not provide the performance necessary for successful long-term durable bonds. Past bond failures, primarily due to inadequate surface preparation, have been a limiting factor in the current use of bonded repairs.
One strategy used to improve the adhesion of the paste adhesive is to introduce a primer coating, also known as an adhesion promoter coating, prior to the introduction of the adhesive bonding material. However, metal bond primers typically contain hazardous components such as volatile organic compounds and hexavalent chromium and as such are severely limited in available use for depot and repair due to environmental and safety concerns.
Another problem with metal bond primers is they typically require a heat cure of at least 250 degrees Fahrenheit. These temperatures, however, can damage the support structure of the aircraft by expansion of water within the honeycomb structure or plasticization of existing adhesive/composite resins. Further, metal bond primers are typically applied over a conversion coating containing hexavalent chromium, and reapplication of conversion coating for repair operations is not practical. The use of metal bond primers is thus severely limited.
Another strategy commonly used to improve adhesion of the paste adhesive is to use an adhesion promoter such as a silane adhesion promoter. The use of silane adhesion promoters, however, involves a two-step process for applying a paste adhesive to the aircraft, wherein the silane monomer is first applied to the metal substrate and subsequently reacted with an additional coating.
Consequently, it would be highly desirable to provide a chrome-free, low volatile organic, protective and adhesion promoting composition for a metal substrate. It is also desirable if the composition could be applied to the metal substrate via a single pack system. It is further desirable if the composition is capable of being cured in ambient conditions.
By reacting water-based bisphenol A epoxy resins with primary aliphatic amine compounds and hydrolyzed alkoxysilane compounds, a water-based oligomer adhesion promoter material may be formed that promotes adhesion of paste adhesives used for structural bonding of metal components.
The present invention offers many improvements over previous paste adhesive systems. For example, because the adhesion promoter is ambiently cured, manufacturing costs associated with elevated temperature curing are eliminated. Further, the elimination of an elevated temperature curing step also may prevent damage to support structures associated with expansion of water and plasticization of the adhesive/composite resins. Further, by improving adhesion between the paste adhesive and substrate, a significant improvement in long-term strength of metal substrates is realized. This is very beneficial for strength requiring applications such as for use on military or commercial aircraft. Also, by introducing the adhesion promoter within fatigue cracks prior to application of the paste adhesive, a significant decrease in crack propagation is realized, resulting in longer usable life of the metal substrates. Further, the present invention does not utilize hexavalent chromium or large amounts of volatile organic compounds, thereby lessening environmental concerns and limiting costs in terms of hazard prevention and cleanup.